This application claims the priority of Japanese Patent Application No. 2001-299093 filed on Sep. 28, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a projection type image display apparatus such as a video projector comprising image display means such as a liquid crystal display device and a digital micromirror device (hereinafter referred to as xe2x80x9cMDxe2x80x9d). More specifically, it relates to a projection type image display apparatus equipped with a switching function for image modes such as the brightness and contrast of projected images.
2. Description of the Prior Art
Various types of projection type image display apparatus have been known depending on differences in image display means for causing light from a light source to carry image information.
For example, a transmission type liquid crystal projector using, as its image display means, a transmission type liquid crystal display panel driven by an image signal has been known. This transmission type liquid crystal projector is configured such that the transmission type liquid crystal display panel is irradiated with light from a light source, so that a part of the light is transmitted therethrough, whereas thus transmitted light is caused to carry image information and converged by a projection lens, so as to project an image onto a screen (Japanese Unexamined Patent Publication No. HEI 10-206816 and the like).
On the other hand, a reflection type liquid crystal projector using a reflection type liquid crystal display panel such as a ferroelectric liquid crystal device (FLC device) as its image display means has been known. The reflection type liquid crystal display panel is configured such that, when a part of light emitted from a light source is reflected, thus reflected light is caused to carry image information (Japanese Unexamined Patent Publication No. HEI 10-48762 and the like).
Also known is a projector using, as its image display means, a DMD comprising a number of light reflection angle variable mirror devices each adapted to change the reflection angle of irradiation light according to the image signal so as to reflect only signal light toward a projection lens system. Using CMOS technology, high-reflectance minute rectangular mirrors (mirror devices) each adapted to change its inclination within the range of about 10 degrees according to image signals are formed on a silicon memory chip, whereby the DMD is made. A video projector using the DMD is configured such that the direction of reflection of light from the light source is controlled by changing angles of mirror devices, such that only desirable reflected light is converged onto a screen, so as to project an image thereon (Japanese Unexamined Patent Publication No. HEI 9-96867 and the like).
While the projection type image display apparatus can be categorized into various types depending on differences in the image display means used therein as mentioned above, it is a common problem to attain projection images which are easier to view.
For example, in order for projection images to be viewed easily in a bright surrounding environment such as one with interior illumination on, their brightness has been increasing. To this aim, improvements in the image display means have been proposed, and various attempts have been made concerning illumination and projection optical systems in order to utilize the light source light effectively. With recent advances in image display means, the advent of DMD in particular, it has become possible to increase the efficiency in utilization of light source light dramatically, thus allowing the projection images to increase their luminance greatly.
Increasing the brightness has made it easier to attain projection images suitable for viewing in a bright room, since the resulting images are easier to view even when their contrast is not so high. However, they will be harder to view in a dark room if their brightness is kept. Therefore, a projection type image display apparatus equipped with an image mode switching function having a changeover switch has come into practical use, which is adapted to switch between an image mode yielding bright projection images suitable for viewing in a bright room and an image mode yielding projection images with a suppressed brightness suitable for viewing in a dark room by operating this changeover switch.
In the above-mentioned image mode switching function, however, the quantity of light emitted from a light source section itself is directly regulated, so as to switch between the image modes, whereby the following problems may occur. Namely, though projection images which are easier to view can be obtained even if their contrast is not so high in the case where the images are bright, the projection images will be harder to view unless their contrast is high in the case where the images are dark. Hence, the contrast is required to increase in the case where the brightness of projection images is to be lowered. In the system in which the quantity of light emitted from the light source section is directly lowered, however, it has been difficult to increase the contrast.
In view of the foregoing circumstances, it is an object of the present invention to provide a projection type image display apparatus equipped with a function for switching image modes according to whether its surrounding environment is bright or dark, which can increase the contrast of projection images while lowering the brightness thereof in the case where the image mode suitable for viewing in a dark surrounding environment is selected.
For achieving the above-mentioned object, the present invention provides a projection type image display apparatus comprising a light source section, an illumination optical system for outputting light from the light source section as predetermined illumination light, image display means for converting the illumination light from the illumination optical system into image display light carrying image information and outputting thus obtained image display light, a projection lens system for emitting the image display light from the image display means and projecting an image onto an image projection surface, and image mode switching means for switching between a first image mode suitable for projection in a bright surrounding environment and a second image mode suitable for projection in a dark surrounding environment according to a setting for image mode switching; the projection image display apparatus further comprising a variable stop mechanism placed at a predetermined position of an optical path from the inside of the illumination optical system to the inside of the projection lens system and adapted to regulate the size of effective luminous flux diameter at the predetermined position; wherein the image mode switching means is configured so as to be able to control an operation of the variable stop mechanism; and wherein the image mode switching is carried out by operating the variable stop mechanism such that the effective luminous flux diameter becomes greater when the first image mode is selected and smaller when the second image mode is selected.
The predetermined position may be set within the illumination optical system.
The predetermined position may be set within or near the projection lens system.
The variable stop mechanism may be constituted by a first variable stop mechanism set within the illumination optical system, and a second variable stop mechanism set within or near the projection lens system.
Preferably, the variable stop mechanism comprises a cooling section for cooling the variable stop mechanism.
The projection type image display apparatus in accordance with the present invention may further comprise surrounding environment detecting means for detecting the brightness of the surrounding environment and setting the image mode switching according to a result of the detection.
As the image display means, a digital micromirror device can be used.